FR2828421A1 - PROCESS FOR COOKING CERAMIC CORES - Google Patents

Abstract

The invention relates to a method of firing ceramic cores, the ceramic core (10) is placed on a charging stand (20), at least one flexible refractory bag (30) containing refractory particles is placed on the core for applying a force on the core (10) towards the oven rack during baking, and then, the ceramic core on the oven rack (20) is heated to a high superambient baking temperature. Heating the ceramic core (10) with the flexible charge bag (s) (30) thereon causes the core to conform to a surface of the charging rack reducing core deformation and improving fuel efficiency. cores within preselected dimensional tolerances. The invention is particularly applicable to the manufacture of blades and fins for gas turbine engines.

Description

Japanese folds obtained by the process according to the claims

preceding.

  The present invention relates to a process for firing ceramic cores to be used for molding

molten metallic materials.

  Most manufacturers of gas turbine engines are in the process of evaluating advanced turbine lift plans made by precision casting (i.e. turbine blades or fins) that have cooling channels air complex to improve the efficiency of the internal cooling of the lift planes or bearing surfaces to allow greater engine thrust and to achieve a sat i sfif health of the bearing surfaces. Internal cooling passages are provided in the molded bearing surfaces using one or more thin ceramic cores configured on the bearing surface positioned in a ceramic casting mold where the molten metal is cast in the mold around the core. After the molten metal has solidified, the mold and the core are removed, leaving a bearing surface molded with one or more internal passages where were before the cores. The ceramic core is typically made using a plasticized ceramic compound comprising a ceramic powder, a binder and various additives. The ceramic compound is injection molded, transfer molded, or it is poured at an elevated temperature into the die or mold. When the green core (uncooked) is removed from the die or mold, it is typically placed between upper and lower racking supports to cool to room temperature before finishing operations and core calibration and baking at a high sintering temperature. The ceramic core in the green state is fired at high temperature (superambient) during one or more steps to sinter and reinforce the core used for the casting of a metallic material, such as a nickel-based or base superalloy cobalt. US Patent 5,014,763 describes a ceramic core in the green state positioned between upper and lower oven supports during firing. The core in the green state may have a deformation resulting from the stresses induced in the core by the molding and / or cooling operations at room temperature. Deformation can be a particular problem of a bearing surface region of the core having a rear edge of a relatively thin cross-section which tends to deform. Consequently, the ceramic cores in the green state can exhibit dimensional variations from one nucleus to another in a phase of execution of the cores. Furthermore, the upper or lower rack supports may come into incorrect contact with the core in the green state so that there are variations in dimensions.

  from one nucleus to another during an emergency phase.

  The object of the present invention is to carry out a process for firing ceramic cores so as to reduce dimensional variations and improve the yield of ceramic cores which respond to

dimensional tolerances.

  In an illustrative embodiment of the invention, a method of firing a ceramic core comprises the steps of placing an uncooked (previously green) or previously cooked ceramic core having a molded core shape on a support in the oven, placing at least one flexible refractory bag containing refractory particles on the ceramic core to apply a force on the core to the oven rack during firing and then heating the ceramic core on the oven rack to a high super-ambient cooking temperature to sinter and strengthen the core intended for

  use when molding a metallic material.

  Heating the ceramic core with the flexible weight or load bag (s) thereon according to the invention helps to make the core conform to a surface of the oven support to reduce dimensional deformation of the core and to improve efficiency cores within preselected dimensional tolerances. Preferably, a plurality of flexible refractory bags of a selected weight are placed on the core in orientations and local isations which have been found to be effective in reducing the dimensional variation of the core relative to printing dimensions. The ceramic core can be treated according to

  the invention in the uncooked state (green) or in the cooked state.

  In a particular embodiment of the invention, the flexible bag comprises a ceramic fiber fabric bag which contains a ceramic chamotte (for example ceramic particles). The bag prevents spreading of the chamotte particles during cooking and prevents a chemical reaction between the chamotte particles and the core. The bag can be configured as a tuLular sock with opposite ends which are closed, after the sock has been filled with the chamoLte, by a metallic wire or ceramic cord

  which can react at the cooking temperature.

  Advantageously, the refractory particles can

  include sintered ceramic particles.

  According to one embodiment of the invention, the ceramic particles comprise aluminum oxide particles which have a dimension determined by sieving

from 3 to 6.

  According to yet another embodiment of the invention, the racking support comprises a surface with a bearing surface contour, and the core has a region configured as a bearing surface received on the surface of the racking support. The nucleus can, for example, have

  a length greater than approximately 15.24 cm.

  According to yet another advantageous embodiment of the invention, at least one bag can be reused for

cook another kernel.

  The invention is advantageous, although it is not limited to this for firing a relatively large ceramic core which has a bearing surface region which tends to deform. By way of example only, the invention is advantageous for firing ceramic cores used for the precision molding of bearing surfaces (for example blades and turbine fins) of gas turbine engines based on the

land and air.

  The invention will be better understood and other aims, characteristics, details and advantages thereof

  will appear more clearly during the description

  Explanatory which will follow made with reference to the appended schematic drawings given only by way of example illustrating an embodiment of the invention and in which: - Figure 1 is a perspective view of a ceramic core with a typical bearing surface to the green (uncooked) or cooked state which can be cooked according to the invention; - Figure 2 is a perspective view of the ceramic core of the bearing surface disposed on a lower charging support, with several flexible load or weight bags thereon according to one embodiment of the invention. The slots and openings shown in the core of Figure 1 are omitted in Figure 2 for clarity; and - Figure 3 is a sectional view of the surface

  bearing on a rack of Figure 2.

  The present invention will be described below, by way of illustration only, in connection with the manufacture of ceramic cores produced by conventional core molding by pouring, by injection molding, transfer molding or other forming techniques. cores o a plasticized ceramic compound is introduced into a matrix or mold. A conventional pouring core molding involves the mixture of one or more ceramic powders (for example alumina, silicon oxide, zircon and / or ziraone powders), a liquid binder such as a liquid binder of catalytically polymerized ethyl silicate, and other additives, pouring the mixture into a matrix and applying pressure until the binder polymerizes / hardens followed by removal of the core from the green state of the matrix. The nucleus in the green state is then subjected to a combustion treatment o the nucleus in the green state is struck with an alcohol flame in order to increase the resistance of the nucleus in the green state before it is subjected high temperature sintering (baking) treatment to sinter and strengthen the core for use in molding a metallic material, such as a nickel-based or cobalt-based superalloy. An injection or transfer molded ceramic core is molded by injecting a ceramic compound comprising one or more ceramic powders (for example an alumina, silicon oxide, ziraon and / or zirconia powder), an organic binder ( for example a thermosetting binder material, a crosslinked thermoplastic or thermoplastic binder material and mixtures thereof) and various additives at elevated temperature in a matrix at temperature of

  super-ambient matrix to form a nucleus in the green state.

  The ceramic powders, organic binders and specific additives for producing the core can be selected from available conventional materials and can be used for this purpose and are not part of this invention. An uncooked ceramic core (in the green state) can be treated according to the invention. In addition, a previously cooked ceramic core can be further processed according to the invention. For example, when a ceramic core poured is subjected to two or more firing treatments, some or all of the firing treatments can be performed according to the invention using the flexible refractory bag (s). For the purpose of illustration and not limitation, the invention will be described below in connection with the manufacture of ceramic cores used for the precision molding of bearing surfaces of gas turbines, such as turbine blades and blades. Such load-bearing surfaces are typically cast precisely using nickel-based superalloys and cobalt-based superalloys, and they are molded to have an equiaxial grain microstructure or a directionally solidified microstructure comprising colonary grains or a single crystal. The invention is particularly useful, but not limited to the manufacture of relatively large ceramic cores used for precision molding of bearing surfaces for land based gas turbine engines where the cores can have lengths.

  larger than about 15.24 cm to 106.68 cm and more.

  Referring to Figures 1 to 3, by way of illustration and not by way of limitation, an illustrative ceramic core in the green state (uncooked) removed from a matrix or mold (not shown) is shown diagrammatically and is configured for use when molding a gas turbine engine blade into a nickel or cobalt-based superalloy. The green core 10 includes a root or base region 12 and a bearing surface region 14. The bearing surface region 14 has a front edge 16 and an edge

  rear 18 of a relatively thin cross section.

  Openings 21a and / or slots 21b of various configurations and dimensions can be made through the core 10 to form oblong walls, rounded bases or bases and other features in the interior of the cast turbine blade, as is well known. The openings and / or slots 21a, 21b are emitted in Figure 2 for clarity. The core 10 has a convex side S1 and an opposite concave side S2, as is well known for cores of the turbine bearing surface. The sides S1, S2 typically exhibit complex surface features such as ribs, plinths, turbulence generators and the like. The rear edge 18 typically decreases to a very thin edge which tends to become

  veil or wave or otherwise deform.

  Referring to FIGS. 2 and 3, the uncooked core (in the green state) 10 is shown in position on a surface 20a for receiving the core, oriented upwards, of a rigid lower charging support 20. The core 10 in the green state can be positioned on the rack 20 when the core is always at a high molding temperature (for example at 32.2 to about 149 C) followed by the withdrawal of the matrix or from the mold, or after the green core 10 has cooled to room temperature. Instead of a green core, the core 10 may include a core which has been previously cooked as part of a multiple cooking program such that it can be used for

ceramic cores poured.

  The rigid lower racking support 20 may comprise a metallic, plastic material (for example a REN plastic obtainable from the company Ciba Geigy), a ceramic material or another relatively rigid / stiff material. The oven support 20 includes a core receiving surface 20a which has a preselected desired profile or contour to which the near side S1 of the ceramic core in the green state 10 can conform. For example, the surface 20a of the charging support is shown in FIGS. 2 and 3 with a bearing surface contour preselected to receive the bearing surface region 4 of the core 10 thereon. The surface 20a of the oven rack typically has a flat outline in that the surface does not have surface details, such as bases, turbulence generating elements and the like which may be present on the region of

bearing surface 14 of the core 10.

  By implementing an embodiment of the method of the invention for baking a core 10 based on silicon oxide, one or more bags of flexible refractory fillers 30 containing refractory particles 32 is / are placed on the side oriented towards the top S2 of the core 10 to apply a generally uniformly distributed force (due to its weight) on the core to the surface 20a of the oven rack during cooking to encourage adaptation of the S1 side of the core to the surface 20a of the oven rack during core cooking in the range of 871 to 1149 C, for example only. Each load bag 30 is flexible so that the bag can adapt to the upper surface of the core 10 to distribute and apply a force (force of gravity) on the core to the surface 20a of the charging support while allowing also gas from the core material to pass through the bag. Each bag 30 can be made from a fabric of woven ceramic fibers (for example silicon oxide or alumina) commercially available as Kaotex fabric from Thermal Ceramics, Augusta, Georgia. A flat sheet of a ceramic fiber fabric which is folded over itself, and the adjacent folded edges of the fabric are sewn together using a silicon oxide or alumina fiber to form a tubular bag or sock. The invention is not limited to the particular ceramic fiber bag or sock since other refractory or ceramic materials can be used to make the flexible filler bags 30 for containing the ceramic particles 32 to cooking temperature and which do not react with the core material during cooking. When firing a nucleus based on silicon oxide, the refractory particles 32 comprise alumina chamotte which comprises sintered alumina particles of a determined size by sieving from 3 to 6 (dimension determined by sieving in accordance with US Standard Sieve). Such alumina chamotte particles typically have diameters of about 0.63 cm. Such alumina chamomile is commercially available from Aluchem, Reading, Ohio. The invention is not limited to the alumina chamotte, since other refractory or ceramic particles 32 can be placed in the weight or load bags 30. The refractory or ceramic particles 32 are chosen so as not to sinter together at the particular temperature and at the time of cooking the core. The flexible tubular load bags 30 are filled manually or by machine with particles 32 of suitable weight, and the open ends of the bags 30 are knotted (closed) using knots of wire or ceramic cord 34 which can resist temperature / cooking time to keep bags closed. Typically, one end of the bag 30 is closed using a first knot 34, and then the bag is filled with particles 32 through the remaining open end followed by closure of the remaining open end using a second node 34. Suitable nodes 34 may include a Ni-Cr thermocouple wire, or a platinum or platinum alloy thermocouple wire which is twisted around opposite ends of the bags 30 to form the end closures. The bags 30 prevent the particles 32 from spreading inside during cooking and prevent a chemical reaction between the

particles 32 and nucleus 10.

  Although FIG. 2 shows three bags 30 positioned transversely and spaced apart on the longitudinal axis of the core 10, these positions of the bags relative to the core are offered simply by way of illustration and not by way of limitation. When practicing the invention, the orientations and locations of the bags 30 on the core 10 are determined empirically for each particular core design to encourage adaptation of the core to the surface 20a of the support. charging and thus reducing the dimensional variation of the core 10 relative to the desired printing dimensions. For example, different core configurations may have different regions which do not inherently fit as tightly as desired to the surface 20a of the carrier due to shrinkage or distortion of the core, and for other reasons. Consequently, during the implementation of the invention, the bags 30 can be oriented transversely to the longitudinal axis of the core 10, as shown in FIG. 2, and / or they can be oriented parallel to the long axis , and / or at any angle relative thereto, as necessary, and determined empirically by systematic experimentation to obtain the adaptation of the particular core 10 to the surface 20a of the charging support. Each bag 30 applies force

  uniformly distributed over the core 10 for this purpose.

  Similarly, the number and weight of the bags 30 (in addition to the orientations and locations of the bags on the core 10) are determined empirically for each particular core concept to encourage adaptation of the core to the surface 20a of the charging support and thus to reduce a dimensional variation of the core 10 relative to the desired printing dimensions. The weights of the bags may be different at different locations on the core 10 for the same purpose. By way of illustration and not by way of limitation, the three bags 30 represented

  can each weigh about 0.68 cg.

  During the execution of an embodiment of the method of the invention, the ceramic core 10 on the charging support 20 with one or more load bags positioned on the core is fired in a conventional oven at a temperature of high superheat firing to sinter the particles of the ceramic core together and to strengthen the core for a

  use for molding a metallic material.

  For firing a green ceramic core based on silicon oxide, the maximum firing temperature of the core can be in the range from 871 to 1149 C depending on the core composition used. Cooking can be done in an air atmosphere in the oven. The core 10 is typically subjected to a conventional cooking cycle comprising a heating step, a step of maintaining the maximum cooking temperature and a cooling step for a period

  aggregate time from several hours to several days.

  The rate of change at maximum firing temperature is typically about 10 to about 75 C per hour for a silicon dioxide-based core,

for illustration only.

  The firing of the ceramic core 10 with one or more flexible weight bags 30 thereon according to the invention encourages the adaptation of the core to the surface 20a of the oven support 20 to reduce dimensional variations of the core and increases the yield. fired ceramic cores which are within preselected dimensional tolerances (photocalque). After cooking, the load bag (s) 30 can be reused for cooking other

nuclei 10.

Claims (13)

RESELL I CAT I ONS   1. A method of firing a ceramic core, characterized in that it comprises the steps consisting in placing a core (10) on a charging support (20), in placing at least one flexible refractory bag (30) containing refractory particles (32) on said core to apply a force on said core (10) to said charging support and to heat said core to   a high ambient cooking temperature.   2. Method according to claim 1, characterized in that several of said bags (30) are placed on said core according to different orientations relatively said core.   3. Method according to claim 1, characterized in that several of said bags (30) are placed on said   nucleus at different locations on said nucleus.   4. Method according to claim 1, characterized in that several of said bags (30) of different weights are placed on said core.   5. Method according to claim 1, characterized in that at least one aforementioned bag (30) comprises a fabric ceramic fibers.   6. Method according to claim 5, characterized in that at least one aforementioned bag (30) has a configuration of tubular bag.   7. Method according to claim 6, characterized in that the opposite ends of said configuration of   tuLulaire bag are closed by a respective knot (34).   8. Method according to claim 7, characterized in that said node (34) comprises a metal wire or a ceramic cord.   9. Method according to claim 1, characterized in that said refractory particles (32) comprise   sintered ceramic particles.   . 10. Process Gelon claim 9, characterized in that the cdramic particles include aluminum oxide particles.   11. ProcAd as claimed in claim 10, characLArisA in that 1eGdiLeG parLiculeG # aluminum oxide onL a   dimension dAterminde by tagiGage of 3 6.   12. ProcAdA according to claim 1, characL6riG in this guise ladiL supporL of charging (20) comprises a surface prAGenLanL a conLour of sticky surface, and said core (10) presents a region (14) configured in suface porlanLe required Gur said suface of said Gupport in the oven. 11. Gelon method of claim 12, characterized in that said core (10) has a shorter length about 15.24 cm.   14. The process as claimed in claim 1, characterized in that it comprises the additional step of reuse of at least one bag mentioned above for cooking another. prAcit @ kernel.   15. ProcAdA Gelon claim 1, characL6risA in that ladi ncyau cAramigue (10} eGL an uncooked nucleus. 16. Procd according to claim 1, caractrisA